The present invention relates to a surface static reduction device for use, for example, in reducing and preferably eliminating static electricity from surfaces to be sprayed with paint. In particular, the present invention provides an improved radioactive gun for the reduction of static.
There are many ways of eliminating static electricity. These may involve the use of high voltage devices which use a corona discharge to generate ionized air, there are so-called passive devices which consist of unpowered arrays of sharp points, there are electrically powered X-ray tube devices which ionize air by emitting low energy X-rays and there are radioactive devices which are generally described as bars, guns or cartridges which use radioactive sources to ionize, air. Devices can be used in combination with each other and in conjunction with, blowers, fans, compressed air lines and the like which guide the ionized air to where it is needed. All the methods seek to produce and direct as many ions as possible to the charged work surface. There they can neutralize unwanted electrostatic charge which may have built up.
The basic design concept and operating principle for radioactive guns and ionizing cartridges is described below by reference to FIG. 1. Devices work by passing air from a high pressure feed line 3 at a high velocity into an input nozzle end 2 of a hollow cylindrical cartridge 1 which is open at the other end with an outlet nozzle 4. Inside the cartridge 1 is placed a source of ionizing radiation 5 which is commonly a metal foil containing the radioisotope polonium-210 which emits alpha particles. This causes the air flowing through the cartridge to become ionized. The air exits the cartridge through the outlet nozzle 4 and is then directed towards a charged surface by the operator of the device. Ions in the air stream are blown onto the surface and/or are drawn towards it by an electrostatic field associated with the charge on the surface and they cause the charge on the surface to be neutralized. Static radioactive guns and ionizing cartridges such as the one represented in FIG. 1 are well known in the industry and such devices using this operating principle have been available for many years.
The main field of application is in manufacturing industry where it is important for certain articles to be kept clean and free from dust and charge during their assembly. An important application is in the paint spraying industry. In this application it is well known that both dust and charge on a surface give rise to a poor quality paint finish and there can be a significant cost associated with rework. The radioactive ionizing gun provides a means of improving the quality of the surface finish by eliminating both dust and charge simultaneously prior to painting. High voltage corona discharge devices are potential fire hazards in this application and they are generally not used by industries which perform paint spraying on safety grounds.
The efficiency of existing radioactive guns and cartridges can be adversely affected where local conditions vary and also due to poor design. Factors which can affect performance include such parameters as the air input pressure, the air flow volume, air turbulence, air cleanliness and particulate content, temperature, humidity, work surface material, geometry and distance from the gun, local electrostatic fields, individual operator training and product age. In poorly designed devices, performance may also be adversely affected due to inefficient ion production, inefficient transport to the work surface and ion losses due to recombination and dispersion in the outside air. The present invention seeks to address the problems encountered with existing radioactive guns and in particular the present invention seeks to provide a radioactive gun which is substantially insensitive to changes in local conditions.
The present invention provides a surface static reduction device for generating a stream of ionized air comprising a cartridge having a chamber with an inlet for communication with a pressurised air supply and an outlet for the stream of ionized air, the chamber containing at least one radioactive source for ionizing air within chamber characterized in that the cartridge is arranged to produce an external stream of ionized air having a core region and a perimeter region with the average ion concentration in the core region being greater than the average ion concentration in the perimeter region.
A cylindrical static reduction device may be provided with a cylindrical radioactive source, the internal diameter of the cylindrical source being greater than 11 mm and less than 23 mm diameter and in which the input air pressure, the inlet diameter and the outlet diameter are matched to produce an internal air density and air velocity contour which causes the production rate of ionisation to be greater in the centre of the chamber than adjacent the walls of the chamber so that the stream of ionized air has an ion concentration in the core region of the air stream which is maximized and an ion concentration in the perimeter region of the air stream which is minimized. In addition the inlet diameter and the outlet diameter may be matched so that the internal air density inside the ionizing cartridge is substantially independent of pressure variation in the compressed air supply line.
Preferably, but not exclusively, the device is designed and operated so that the internal air density inside the cartridge is such that the maximum path length of alpha particles from the source is between about 0.55-0.85 (preferably 0.65-0.8, more preferably about 0.75) of the internal diameter of the source, (or if the source is planar, that fraction of the average height of the air space above the source). This produces an ion distribution in which the ion cloud from opposite sides of the cartridge overlap in the middle to produce a core region of higher (i.e. about double) ion concentration. The larger the cartridge diameter, the longer the alpha path length needs to be before there is overlap in the middle. The optimum air density is lower for large cartridge diameters.
Because of the need to balance ion concentration with air pressure in dependence on the diameter of the cartridge a practical limit arises for the useful range of internal diameters for such devices of between about 12-22 mm. The 12 mm devices need to be designed to operate at high internal pressure for optimum performance whereas the 22 mm diameter devices need to be designed to operate at low internal pressure for optimum performance.
In order to ensure the optimum internal operating pressure (i.e. air density) is achieved for any given cartridge diameter more usually in the range 12-22 mm the ratio of the internal diameter of the radioactive source to the output nozzle diameter is important. In a preferred embodiment, optimum performance is achieved when this ratio is in the range 2.5-4.5, preferably 3-4, more preferably 3.5.
In a second preferred embodiment the inlet nozzle diameter and the outlet nozzle diameter are matched so that the inlet nozzle has an air flow resistance which is greater than the air flow resistance of the output nozzle. The air inlet nozzle acts as the primary barrier to air flow through the device. When the velocity at the inlet is close to supersonic (as it usually is in practical conditions of use) the air input is said to be xe2x80x9cchokedxe2x80x9d. This causes the internal air density of the cartridge to be substantially independent of the air input pressure of the high pressure feed line. In other words, changing the input pressure does not substantially alter the air flow through the input nozzle. This enables the device to operate at optimum efficiency over a wide range of possible input pressures. This is achieved and optimized with the current invention when the ratio of the diameters of the output nozzle to the input nozzle is in the range 1.2-1.4, preferably 1.3. A representative set of workable design parameters for practical devices is summarized in the table below.
Local air velocity and velocity gradients inside the cartridge are important in determining where the local build up of ionization occurs, and how efficiently the ions produced are expelled from the device. Ion concentration is a function of both the ion production rate (due to source activity, location and ion overlap) and the local air flow volume per unit time at the point in question. Where the air velocity is high, the instantaneous ion concentration is low and vice versa due to the local rate of mixing. It is preferable to ensure that the air velocity in the perimeter region (i.e. adjacent to the source) is maximized so that the ion concentration in this region is low.
All static reduction devices lose ions due to recombination. This can occur inside a device due to collisions with internal surfaces or due to annihilation by collisions with oppositely charged ions. The amount of recombination is a complex function which depends on ion concentration, air flow velocity and turbulence and proximity to internal surfaces and on the total surface area available for recombination. In addition to this ions are lost outside the device due to dispersion and mixing with the outside air.
With the present invention, ion recombination losses are reduced due to the inclusion of novel design features which maximize the air flow velocity in the perimeter region of the cartridge. This substantially reduces the time for ion collisions and recombination to occur on the internal cartridge walls.
In a preferred embodiment an air inlet adapter is fitted to, or is integral with the inlet nozzle to improve the performance which prevents high velocity air from travelling straight along the central axis of the cartridge. The adapter deflects the air away from the central axis and towards the perimeter region where the velocity is then maximized.
In an alternative preferred embodiment, an inlet nozzle and air inlet adapter are, in practice, combined as a unitary item to provide both high air resistance compared with the output nozzle and also to deflect air from the central axis of the cartridge.
In an alternative preferred embodiment the at least one radioactive source is mounted so that it is substantially concentric with the chamber and divides the chamber into two air paths an inner ionizing air path and an outer path.
Thus, the surface static reduction device may be in the form of two concentric tubes open at both ends into which air is input at one end and in which the device contains a radioactive source inside the inner concentric tube which ionizes the air as it passes through the inner tube whereby air passing through both the inner tube and the outer tube recombines to form a single air stream which passes out of the device to form a stream of ionized air in which ions are predominantly located in the central core of the air stream. Alternatively the inner concentric tube may be replaced by a source and holder; the holder connects the source to the cartridge wall on support mountings (e.g. legs) so that air can pass in front of and behind the source without the need for a separate inner cartridge body to be located inside the chamber.